Saturday, 16 October 2010

Exercise 3: Histogram

The purpose of this task is to familiarise myself with histograms by relating each one to an image. Histograms should be used twice during workflow, once on the LCD of the camera and again in post production.

Shoot 3 scenes of low, average and high contrast and for each of these scenes shoot 3 versions, correctly (or averagely) exposed then one that is one stop darker and another one stop brighter. Resulting in 9 seperate images and histograms to be analysed.

Standard f stop number scale is :-

1.0, 1.4, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128

Each image should then be opened in Photoshop with the highlight and shadow clipping warnings displayed. Screenshots are to be taken along with relevant notation of observations made.

High Contrast Image

The following shots were taken in my back garden on a cloudy day using ISO 400 and a Cloudy WB. The 1 stop gaps between exposures were made by altering the fstops as I am more confident with 1 stop gaps with aperture rather than shutter speed. The correct exposures were taken @ f5.6 the underexposed @ f8 and the over exposed @ f4.

I placed a white statue against a shaded bush hoping this would create enough of a contrast.With a high contrast image the values are spread across the histogram from one end to the other.

f5.6 JPEG

Screen shots of both RAW and JPEG files were taken to see if there was much of a comparison between the two and also to be able to display highlight and shadow clipping in RAW.

To my eye there only seems to be a very slight variation.

f5.6 RAW
Because of the nature of this shot and the fact the sun peeked out whilst I was shooting there are few highlight warnings on the uppermost leaves and a little on the statue. In the densest part of the bush there are also a few shadow warnings, these indicate that there are areas within this image where there is pure white and pure black.

f4 RAW
With the overexposed image the values on the histogram have moved to the right and there is a marked peak at the very end of the scale. There are only a few very indescernable shadow warnings whilst the highlight clipping has increased.

f8 RAW

With the underexposed image the values again have shifted but this time to the left with there being nothing touching the righthand side at all. This time the balance of clipping has also changed to display more areas of deep shadow. There are a few highlights due to the sun falling on the leaves and middle of the statue.


Low Contrast Image

A sunnier day but with nothing readily available which was offering me a good low contrast image/histogram I resorted to shooting the clouds in the sky.

f5.6 RAW

With a low contrast image the values are more squeezed together in the histogram. There are no absolute blacks or whites and no clipping was displayed.

f4 RAW

With the over exposed image although the values shift to the righthand side they still do not touch the end and again there are no clipping warnings.

f8 RAW

Viewing the histogram for the underexposed umage you can see that the values shift to the lefthand side but yet again do not touch the very end of the scale. Once more there are no highlight or shadow clipping warnings.

Average Contrast Image

f5.6 RAW

 For an average contrast image the histogram usually the full range of values are represented and the graph gradually peaks in the middle with a gentle slope back down the other side. There was just a minimum of highlight clipping on the pot and a tiny amount of shadow clipping under a few leaves.

f4 RAW

On the overexposed image the values shifted a little to the right and the highlight clipping only increased slightly.

f8 RAW

The histogram for the under exposed image definately show the values shifting to the left and the shadow clipping has increased in the shaded areas.
I enjoyed this exercise more than I thought I would. It was quite a challenge to obtain good examples of each histogram which surprised me. Some of the situations I thought would provide me with high contrast or average contrast histograms in fact didn't! Which in effect emphasises the need to regularly check the camera histogram because what your eyes see the camera dosent and this may well effect the exposure settings we choose at the outset.

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